A PRELIMINARY THORON SURVEY IN THE UPPER MIDWEST
.proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Daniel J. Steck
Physics Department, St. John's University
Collegeville, MN 56321

ABSTRACT
Although thoron (2201Zn)was recognized as a potential source of radiation exposure in
homes twenty yearsago, only a few homes in the United States have been measured for
thoron. Early indoor surveys suggested that thoron concentrations would generally be
low, highly variable, and make a poor surrogate for thoron pro eny dose. Howeyer,
recent reports of significant thoron interference with some radon (522Rn) detectors have
renewed interest in thoron. In the upper Midwest, a region where three of the North
American radon-lung cancer studies took place, basement living spaces near thorongenerating soil are common raising the possibility of thoron interference in those studies.
Passive, integrating, radon-thoron discriminating detectors (ATDs) and nondiscriminating detectors were developed and exposed for 90 days in 76 Minnesota homes
and 19 eastern Iowa homes-at interior locations where ATD radon measurements are
routinely made. The overall thoron and radon distributions are lognormal with geometric
mean (geometric standard deviation); and arithmetic mean of 20 (1.8), 35 Bq m-3
respectively and 140 (2.6), 210 Bq m"3.Thoron and radon were correlated, especially for
basement measurements where both isotopes had higher concentrations than on first
floors. In the nondiscriminatory ATD, the median percentage of tracks attributable to
thoron was 17%. This contamination was enough to cause some ATD results to be false
positives for the current radon action level in some of the homes measured. The thoron
interference would create a contamination of the radon-related dose estimate that
averages about lo%, assuming current equilibrium ratios and dose model estimates are
correct.

INTRODUCTION
During the 1 9 8 0 ' ~the
~ discovery of unexpectedly high concentration of radon gas (%n)
in some US homes led to a rapid expansion of residential radiation measurements. In
addition to
a few measurements were made for the presence of ^Rn, the radon
isotope commonly called thoron (Schery 1985, 1990). Thoron, like radon, can pose a
health risk because it can generate progeny that can deliver dose to lung and other tissues.
However, the short half-life of thoron limits its transport from sources like soil and
building materials to indoor living spaces. Thus, it was generally believed that thoron
concentrations indoors would be low and have large spatial variation making thoron
exposures difficult to assess (Nero 1988). However, one decay product of thoron, ^ ~ b ,
can accumulate significant potential alpha energy concentrations (PAEC) even at low
thoron concentrations because it has a long half life. These characteristics of the thoron
decay chain led early researchers to focus their surveys on 'I2pb rather than thoron to
better quantify the potential radiation dose related to thoron. They found that thoron can
generate a significant fraction of the total PAEC in homes and can be an important
fraction of the dose delivered to the lungs (Schery 1985, 1990, Dudney et al. 1990, Martz
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. 75
www.aarst.org

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
et al. 1990, Tu et al. 1992). However, since few measurements of thoron or its progeny
have been made in homes since the early 1990s, its spatial and temporal distribution
indoors is largely unknown.
Thoron can interfere with the detection of radon in detectors that do not use energy
discrimination and that allow rapid gas entry into their sensitive volume. Scintillation
cells, ion chambers, and alpha track registration detectors (ATDs) are susceptible to
thoron interference. One type of commercially-available ATDs was found to be almost
equally sensitive to thoron and radon (Pearson and Spangler 1990, Tokonami et al. 2001).
However, thoron has been generally ignored both as a source of dose and as an
interference in US home radon measurements.
In recent years, interest in thoron has been refreshed with the discovery of elevated
thoron indoors in China and the possible effect that thoron interference may have had on
some radon-lung cancer epidemiology studies. (Shang et al. 1997, Wiegand et al, 2000,
Tokonami et al. 2004, Yamada et al. 2005) Many of the epidemiologic studies in North
America used an ATD model that is susceptible to thoron interference (Tokonami et al.
2001, Field et al. 2006).
The primary purpose of this work is to assess the interference of thoron with the typical
radon measurements made in the Upper Midwest for home radon assessment or
epidemiological studies.

METHODS
These measurements used selection and exposure protocols that were similar to the three
radon-lung cancer studies from the region (Field et al. 2006). They also conformed to
EPA guidance for ordinary long-term home radon measurements. Detectors that were
sensitive only to radon and detectors that were sensitive to both radon and thoron were
exposed side-by-side in frequently occupied living spaces for 90 days.

Samnlin~sites and measurement protocols
To date, 76 homes in Minnesota and 19 homes in Iowa have been measured for thoron.
The selection and sampling criteria were slightly different in the two states. In
Minnesota, towns for the survey were selected on the basis of their surface equivalent
thorium concentration (eTh). The data presented by Darneley et al. 2003, shows that the
equivalent thorium surface content of the upper Midwest ranges from extremely low to
roughly two thirds of the maximum observed in North America. The surface radiation
measurements from the NURE program were aggregated over each Iowa and Minnesota
county to estimate the geographic thoron potential of sites in the upper Midwest (Duval
and Riggle, 1999). While some towns with low eTh were included, the emphasis in
Minnesota was on towns with high eTh since this better reflected the eTh content in
Iowa. Figure 1 shows that the sampled locations included high and low eTh contents.
Inside those Minnesota regions homes were selected randomly from phone directories
and enrolled via telephone. In Iowa, the high radon houses were selected because their
Copyright 02007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

76

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
primary study required elevated radon concentrations. All Iowa houses were in the same
county and are estimated to have the same eTh.

Figure 1: Map of the estimated thorium content of the surfaces of Minnesota and Iowa
with the location of the selected houses shown. The number of samples in each
concentration category is shown next to the legend.
In Minnesota only one measurement was made in each house while in Iowa multiple
measurements were made in each house. All measurements took place in living spaces. In
Minnesota, the measurement was made in the lowest floor where someone spent 10 or
more hours per week. Homeowners placed the detectors following directions that were
based on EPA placement protocol. In Iowa, the detectors were placed and retrieved by a
trained technician following EPA protocol. All measurements were long-term, 90 days or
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

77

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
more. The measurements usually took place in the heating season but many extended into
the non-heating season.

Radon and Thoron measurement devices
Pairs of high sensitivity alpha track detectors, which had been developed for measuring
atmospheric radon concentrations, were adapted to discriminate between radon and
thoron (Steck et al. 1999.) In this system, one of the ATDs is enclosed in a thin
polyethylene film to exclude thoron (closed detector) while the other ATD has a coarse
metal grid and cloth coveringthe large entry port allowing for rapid gas ingress. The
detector's radon and thoron response was measured through multiple exposures in two
chambers in our laboratory. The radon response of the open and closed ATDs were
determined in a room which contains no measurable thoron but can have constant radon
concentrations of about 1 k kBq m'3 [25 pCi/L] . The radon concentration in the room is
measured hourly with calibrated CRM (Durridge RAD-7) The radon response of the open
ATD has also been calibrated in national radon tests for more than 10 years. The smaller
thoron chamber has a mixture of 95% thoron and 5% radon. We confirmed the thoron
calibration of the open ATDs through an intercomparison with National Institute of
Radiological Sciences in Chiba, Japan. The closed detector has a response of 5.7 tracks
(kBq hr m'3)"to radon and less than 0.05 tracks
(kBq hr m'3)" to thoron. The
open detector has a thoron response of 5.2 tracks cme2 (kBq hr m'3)'1 and a radon
response of 5.7 tracks c m ' (kBq hr m ) " .
The statistical uncertainty in the individual ATD track density is approximately 10%.
Since The closed detector's track density depends on radon exposure alone while the
open detector's track density reflects the radon and thoron. Hence, the thoron exposure
uncertainty depends on the radon exposure as well as the thoron exposure. For example
for a 90 day exposure, the thoron LLD at 37 Bq m'3 [l pCi/L] was 10 Bq m'3 [0.2 pCi/L
while at a radon concentration of 400 Bq m'3 [I 0 pCi/L] the thoron LLD was 100 Bq m"
[2 pCi/L].

i

RESULTS
Table 1 shows a summary of the results. Some caution must be used in interpreting and
comparing results because site selection was not completely random. Thus the
distributions may not be representative of the entire region. Some of the distributions are
neither normal nor lognormal.

Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

78

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Table 1. The distribution of indoor radon ( ~ nand
) thoron ( ^ ~ n ) measured in selected
Minnesota and Iowa homes.
Data Set
N
222~n
"'Rn
220~n
'"Rn
G M (GSD)
~
Mean1
GM2' (GSD)
~ean'
All
Basement
First +

MN
Basement
First +
IA
Basement
First +

114
36
74
76
23
51
38
13
23

210
320
165
145
230
110
340
460
290

[5.7]
[8.5]
[4.4]
[3.9]
[6.3]
[2.9]
[9.2]
12.41
[7.8]

140 (2.6)
210 (2.6)
1 1 5 (2.4)
90 (2.4)
140 (2.6)
80 (2.2)
300 (1.6)
430 (1 -4)
260 (1.6)

35
55
29
39
75
25
30
25
35

[LO]
[IS]
[0.8]
[1.1]
[2.0]
[0.7]
[0.8]
[0.7]
[1.0]

20 (1.8)
21
10

28 (2.1)
27
8
17 (2.6)
15 (2.5)
17 (2.0)

'units arc Bq m"' [pCi/L]
'Lognormal distribution; Geometric mean and (geometric standard deviation).
'When neither normal nor lognormal distributions are appropriate only the median value is given

Figure 2 shows that the overall radon and thoron distributions were close to lognormal.
This set of houses has elevated radon concentrations compared to the regional average
(Steck 2005, Field et a1 2000).

Figure 2 Probability plots of thoron and radon samples.
Since thoron concentrations were calculated from the track density difference
between the open and closed ATDs, elevated radon concentrations created high LLDs for
thoron concentrations. The avera e LLD for individual open ATDs was 79 Bq m'3 [2.1
pCi/L] with a range from 9 Bq m' [0.2 pCi/L] to 460 Bq m'3 [12 pCi/L]. Figure 3 shows
the distribution of the ratio of thoron-created tracks to radon-created tracks in the open,
nondiscriminatory ATDs

9

Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.ore

79

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006

Tn to Rn Track ratio

Tn to Rn Track ratio

(b)

(a)

Figure 3. The distributions of the ratio of tracks caused by thoron to those caused by
radon in detectors exposed side-by-side. Negative values are not plotted in (b).
Figure 4 shows that, in this sample, there is little correlation between the local eTh
content of the soil and either indoor thoron or radon concentrations. Note that the samples
from the low eTh region had both low thoron and radon.

Figure 4. Thoron and radon concentration dependence on local surface soil thorium
content.
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.ors

80

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
Figure 5 shows the correlation between indoor radon and thoron in this region. The
correlation is strongest in the Minnesota, particularly in basements.

A.

Radon: ' " ~ n ( ~ ~ l r n ~ )

Figure 5 Thoron and radon concentrations at the indoor sites

DISCUSSION
Thoron-radon measurement interference
The primary purpose of this study was to gauge the interference that thoron may cause in
typical radon measurements done for home risk assessment or as part of an
epidemiological study. The magnitude of the interference will depend on the relative
thoron to radon sensitivity of the technology. In the US, most long-term measurements
are made with ATDs; at least one widely-used model is known to be quite sensitive to
thoron.
For ATDs, the ratio of the track difference between the open ATD and the closed ATD
tracks serves as a measure of interference. Using this measure, the median contribution of
thoron to the open ATD track density was 17% with a range from -30% to 230%. (The
thoron to radon concentration ratios would only be 5% higher since the relative
sensitivity of the. open detector was only 5% higher for radon.) Other ATD models or
devices with different sensitivity would show a different interference statistic. For
example, using the RADTRAK sensitivity reported by Tokonami (Tokonami et al. 2001),
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

81

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
the interference ratio statistics for the sites in the present study would have a median of
13% thoron contamination with a range from -20% to 150%.
The additional signal created by thoron may have a significant impact on home
measurements in regions where the median home's radon may be near the EPA action
level. Five homes had radon concentrations below the action level but ATD results above
the action level due to thoron contributions.

Indoor thoron concentration distributions
Thoron concentration distribution parameters (median, variation) give a general measure
of indoor thoron in this region. Thoron concentrations were generally higher in the
basement, particularly at the Minnesota sites. The thoron concentrations at individual
sites suffered from substantial uncertainties when the radon concentration was high and
the thoron concentration was low. Hence accurate individual site thoron concentrations
are only available when thoron concentrations were about the same or higher than the
radon concentration. Thoron concentrations exceeding the LLD were observed at 20% of
the sites, 11% in Iowa and 23% in Minnesota. Sixty percent of those sites were first
floors. The maximum thoron concentration observed was 510 Bq m'3 [13.7 pCi/L] in a
basement recreation room.
Few indoor thoron measurements have been reported in the literature; most research has
focused on thoron progeny measurements. Schery, who reported thoron measurements at
25 indoor sites in 5 states, observed lower and more diverse concentrations (Schery
1985). The sites were mainly in southern and western United States where grab sample
measurements were made, presumably during the day. The geometric mean and
geometric standard deviation was 6 Bq m " and 4.2 respectively for that survey compared
to 20 Bq m'3 and 1.8 for this upper Midwest survey.
Following the discovery of some very high indoor thoron regions in China (Shang et al.
1997, Wiegand et al. 2000, Yamada et al. 2006), new reports are beginning to appear in
the literature that give a picture of indoor thoron in a variety of climates and housing
construction (Kim, Shang , and Martinez). The ranges of concentrations in these reports
overlap with those found in the upper Midwest.

Thoron correlations
The estimated equivalent thorium soil content (eTh) does not appear to be a useful guide
to elevated indoor thoron concentrations since the regression coefficient was low
(R-0.02). The highest correlation (R2 -0.1) was found in Minnesota basement sites.
There have been few reports about correlations between indoor radon and thoron. Schery
reports a good correlation between the two gas concentrations; R~-0.5 (Schery 1985,
1990 Li and Schery 1992) which is comparable to the correlation found in this study;
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

82

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
R2-0.4. The radon-thoron correlation is best in Minnesota basements; R2-0.8. However,
poor correlation was observed between radon and thoron in houses with soil walls in
Gansu Province China (Tokonami et al. 2004, Shang et al. 2005, Yamada et al. 2006). In
those houses, the radon concentration was generally lower and the thoron concentration
was higher than in the upper Midwest houses. The difference may reflect differences in
building materials and radonlthoron entry mechanisms in the two regions.
Simultaneous, time integrated measurements of both thoron and its progeny are difficult.
No surveys of correlations between the two have been reported in the US. A report of
recent work suggests that a central estimate of the equilibrium ratio between thoron and
its progeny in US homes may be 0.02 (Harley and Chittaporn 2006). This value is similar
to one reported for homes in China (Tokonami et al. 2004, Yamada et al. 2006) but much
less than the value given in UNSCEAR 2000. Since the correlation between thoron and
its progeny at individual sites is not good, the ratio is only useful for making estimates for
a "typical" house.
Im~licationsfor epidemiolo~icalstudies
Epidemiological studies with small sample size depend on good correlation between the
surrogate that they measure and the disease risk factor. In the case of radon and lung
cancer, one would like to have a good surrogate for the energy delivered to the lungs of
each individual over an extended period in the past. Other factors that interfere with the
surrogate measure - dose correlation tend to obscure the trend between surrogate and
disease (Steck and Field 2006).
Thoron could interfere in several ways. If a thoron insensitive device is used to measure
the radon, then the dose from thoron, which presumably carries some risk of causing the
disease, is not included in the surrogate measurement. As an example, the retrospective
radon reconstruction technique which uses implanted 2 ' 0 ~ as
o a surrogate is insensitive to
thoron progeny.
Most epidemiological studies have used ATD tracks, interpreted as radon exposure, as
the dose surrogate. How much of a problem might thoron be in this case? For illustration,
let's assume that the current estimate for the thoron equilibrium factor (-0.03) is correct,
the microdosimetry models for both radon and thoron progeny are reasonably good, and
that the ATDs used for measurements are roughly equally sensitive to both thoron and
radon. Then, for the radon and thoron concentrations in the upper Midwest survey, the
thoron contamination would represent an average 10% error in the radon-related dose
estimate based on a nondiscriminatory ATD. But in regions or houses where either thoron
concentrations are higher or radon concentrations are lower, thoron may be more of a
problem. For example, some North American epidemiological studies have been done in
regions where the radon concentrations are low (Field et al. 2006). If the thoron levels
observed in the present study region, where the typical radon concentration is -100 Bq
m3, were present in those other epidemiological study regions, where radon
concentrations are a factor of 2 to 5 lower, then the possibility for serious thoron
interference with the studies' analysis is possible.
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst .orq

83

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006

Substantial concentrations of thoron were found in upper Midwest living spaces. Thoron
generated tracks added 17% on average to the tracks from radon in the ATD used in this
study, Thoron interference has the potential to cause significant problems for some
sensitive radon assessment applications. However, the magnitude and distribution of
time-averaged thoron and thoron progeny indoors needs to be studied in a wider sample
of houses.

ACKNOWLEDGMENTS
I wish to thank Kainan Sun for the placing and retrieving the detectors in Iowa and Shinji
Tokonami for exposing the detectors to known thoron concentrations. This work was
made possible, in part, by grant number ROl CA85942-0 1Al from the National Cancer
Institute, National Institutes of Health and grants from the Minnesota Department of
Health (MDH) through the SIRG program of the US EPA. This report is solely the
responsibility of the author and does not necessarily reflect the official views of the NCI,
NIH, MDH or EPA.

REFERENCES
Darneley AG, Duval, JS, Cars, JM. The surface distribution of natural radioelements
across the USA and parts of Canada: a contribution to Global Geochemical
Baselines. 6th International Symposium on Environmental Geochemistry,
Edinburgh, Scotland, 2003. http://gsc,nrcan.gc.cdgammddis~index~e.php
accessed 811712005
Dudney CS, Hawthorne AR, Wallace RG, Reed RP. Radon-222, 222Rn Progeny, and
220Rn Progeny Levels in 70 Houses. Health Physics 58~297-31 1; 1990.
Duval JS, Riggle FE. 1999: Profiles of gamma-ray and magnetic data from aerial surveys
over the conterminous United States: US Geological survey Digital Data Series
DDS-3 1, release 2 , 3 CD-ROM.
Field RW, Steck DJ, Smith, BJ, Brus CP., Neuberger JS., Fisher, EF, Platz CE,
Robinson, RA, Woolson RF., Lynch CF. Residential Radon Gas Exposure and
Lung Cancer: The Iowa Radon Lung Cancer Study. American Journal of
Epidemiology. I 5 1( 1 1), 1091-1 102; 2000
Field RW, Krewski D, Lubin JH, Zielinski JM, Alavanja M, Catalan VS, Klotz JB,
L6tourneau EG, Lynch CF, Lyon JL, Sandler DP, Schoenberg JB, Steck DJ, Stolwijk
JA, Weinberg C, Wilcox HB. An overview of the North American residential radon
and lung cancer case-control studies. Journal of Toxicology and Environmental
Health, Part A, 69: 1-33; 2006
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst .orq

84

Proceedings of the 2006 International Radon Symposium
September I7 - 20,2006
Harley N and Chittaporn P. Radon and thoron equilibrium factors: calculation and
experimental values. Health Physics 9O(Supplement), S 103; 2006
Li Y, Schery SD, Turk B. Soil as a source of indoor 220Rn. Health Phys. 62~453-457;
1992.
Martz D, Falco R, Langner GJ. Time-Averaged Exposures to 220Rn and 222Rn Progeny
in Colorado Homes. Health Physics 58:7O5-713; 1990.
Nero AV, Radon and its Decay Products in Indoor Air: An Overview. In: Nazaroff WW,
Nero AV, eds. Radon and its Decay Products in Indoor Air. New York, NY: John
Wiley & Sons; 1988: 1-53.
Pearson MD, Spangler RR. Calibration of Alpha-Track Monitors for Measurement of
Thoron (220Rn). Health Physics 60:697-7O I ; I 99 I .
Schery SD. Measurements of Airborne 212Pb and 220Rn at Varied Indoor Locations
within the United States. Health Physics 49: lo6 1-1 070; 1985.
Schery SD. Thoron in the Environment. Journal of the Air and Waste Management
Association 40:493-497; 1990.
Shang B, Wang Z, Iida T, lkebe Y, Yamada K. Influence of 220Rn on 222Rn
measurement in Chinese cave dwellings. In Radon and thoron in the human
environment, eds. A. Katase, and M. Shimo, pp. 379-384; 1997. Singapore: World
Scientific.
Shang B, Chen B, Gao Y, Wang Y. Thoron levels in traditional Chinese residential
dwellings. Radiat Environ Biophys 44: 193-199; 2005
Steck DJ, Field RW, and Lynch CF. Exposure to atmospheric radon. Environmental
Health Perspectives; 107, 123-127; 1999.
Steck DJ. Residential Radon. Risk Assessment: How well is it working in a high radon
region?. 2005 International Radon Symposium, Sari Diego CA. September 2005
Steck D, Field RW. Dosimetric challenges for residential radon epidemiology. Journal of
Toxicology and Environmental Health, Part A, 69t655-664; 2006
Tokonami S, Sanada T, Yang M. Contribution from thoron on the response of passive
radon detectors. Health Phys. 8O:6I2-6 1 5; 200 I
Tokonami S, Sun Q, Akiba S, Zhuo W, Furukawa M, lshikawa T, Hou C, Zhang S,
Narazaki Y, Ohji B, Yonehara H, and Yamada Y. Radon and thoron exposures for
cave residents in Shanxi and Shaanxi provinces. Radiat. Res. 162:390-396; 2004
Tu KW, George AC, Lowder WM, Gogolak CV. Indoor Thoron and Radon Progeny
Measurements, Radiation Protection Dosimetry 45557-560; 1992.
Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc. 85
wwvmrst .orq

Proceedings of the 2006 International Radon Symposium
September 17 - 20,2006
United Nations Scientific Committee on the Effects of Atomic Radiation. 2000. Sources
and Effects of Ionizing radiationy Report to the General Assembly of the United
Nations with Scientific Annexes* United Nations sales publication E.00.1X.3y New
York.
Wiegand J, Feige SyQuingling X ySchreiber UyWieditz Ky Wittmann Cyand Xiarong L.
Radon and thoron in cave dwellings (Yan'an, China). Health Phys. 78~438444;
2000.
Yamada Y, Sun Q, Tokonami S, Akiba S, Zhuo WyHou C, Zhang S, Ishikawa T,
Furukawa M, Fukutsu K, Yonehara H. Radon-Thoron discriminative measurements
in Gansu Provincey China*and their implication for dose estimates. Journal of
Toxicology and Environmental Health* Part Ay69:723-734; 2006,

Copyright 0 2007 by the American Association of Radon Scientists and Technologists, Inc.
www.aarst.orq

86